In terms of retinal imaging, in the past it has been the practice to utilize a strobe as a light source for a retinal camera, with the strobe being pulsed as quickly as one and a half times per second to permit the formation of a so-called angiogram for the detection of retinal damage, primarily due to blood leakage. In order to determine the locus of the leak, a patient is injected with dye that goes through the bloodstream in about 30 seconds, at which point it arrives at the back of the eye. At the time that the dye arrives at the back of the eye, a retinal camera is started to capture sequential photographs at roughly one and a half pictures per second so that one can obtain images of the progression of the dye as it passes through the blood vessels in the retina.
The purpose of providing sequential photographs is to be able to ascertain where a leak occurs in the eye, which to view the progression of the dye. Thus, with eye bleeding one needs to be able under normal circumstances would be viewed as a large patch of blood absent being able to ascertain where the leak is coming from and then where it spreads out to so that one can go in with a laser and seal just a small portion of the retina to stop the leak.
In order to obtain good retinal images, one needs to have sufficient illumination and for various retinal cameras with associated optical efficiencies and various fields of view, one requires illumination from 10 to several hundred watt seconds of white light. For this purpose, xenon strobe lamps are used, which have a temperature rating in terms of color that one has to correct for in order to obtain a white light image.
In normal practice the photographer decides at what point he or she wishes to take a picture and with a foot pedal or other button activates the camera. Once activated, the flash goes off and the picture is taken. Note that both manual and automatic activation of the strobes have been used in the past.
The most popular retinal camera is one made by Carl Zeiss, which was originally a film camera that dates back to the 1920s. The Zeiss FF-1 is a fairly old device, the major problem of which was obtaining enough flash output, namely enough power to reliably obtain a flash every 1.5 seconds. In the older cameras, a simple unregulated step-up transformer and unregulated capacitors were used to directly pump the flash lamp. Because of the variation of the load and the flash lamps utilized, the voltage applied to the flash varied significantly, which varied the flash lamp output from one strobe pulse to the next. Thus one could fire the strobe twice and one would not necessarily obtain the same exposure due to the unregulated transformer and the unregulated capacitors. Since the capacitors were unregulated, there could be as much as 30% variance with each shot.
Carl Zeiss in later years tried to solve these problems, finally utilizing semiconductor switching. These later models required an exceptionally large transformer that could generate more voltage and handle more current than the predecessor models. The result was that in the later Zeiss retinal cameras, Zeiss was able to reliably provide capacitor discharge at a regulated voltage.
The problem with these power supplies when used to power xenon flash lamps was that the power supply was relatively large and cumbersome, sometimes weighing in excess of 60 pounds and having an outside dimension of 4×5×3 feet.
Moreover, the bottleneck for all of the Zeiss power supplies was the step-up transformer.
Moreover, with transformers it is difficult to regulate the maximum voltage output. Typically for retinal camera applications the voltage should not exceed 500 volts. If the 500-volt output was exceeded due to variable loading, it was possible to blow up the capacitors used in the strobe bank, typically because even the best of the capacitors were and are rated for a maximum of 500 volts. Also, while rare, the xenon tube could also be damaged due to excessive voltage.
More importantly, one of the failings with the Zeiss power supplies was the fact that the power supplies would not be able to recharge the capacitor bank sufficiently fast to provide one flash per second. The problem in reducing the flash interval from 1.5 seconds to 1 second with a maximum strobe output was the advent of digital cameras. Utilizing film, one could obtain the one-second intervals for the strobes because one could use less than full power in the flash lamps. However, with the use of digital cameras having increased resolution came the need for higher flash outputs. It is noted that with higher resolution one has smaller pixels; and with smaller pixels, the individual pixels do not see as much light as the larger pixels. Thus there is a direct correlation between resolution and sensitivity. Although 11-megapixel cameras are now available, the standard retinal camera is a 6-megapixel device that requires the full 500 volts across the strobe to produce the required maximum flash output.
Thus, with transformer-based power supplies, since the resolution increases with the number of pixels in the camera, the higher the output of the strobe had to be, the longer would be the recharge time for the capacitors. As a result, for higher-power strobes it was virtually impossible to obtain one-flash-per-second strobing.
Not only was it deemed desirable to eliminate the transformer and to reduce the size and weight of the system, there was a problem with increasing the efficiency and, more importantly, lowering the electromagnetic interference/electromagnetic compatibility (EMI/EMC) that was the result of utilizing transformer-based power supplies.
Moreover, with transformers there were only a limited number of methods for controlling the charging of the strobe capacitor bank and the output of the bank.